The iPad mini 4 is slightly taller than the mini 3 so the front cover for the mini 3 will be odd on the mini 4. The cover that goes on the back of the mini 3 will not fit on the mini 4. You'r better off just buying the mini 4 smart cover and silicone case.

Yes you can. But price of those two totals in 1/4 of the price of the iPad Mini itself. For previous models (iPad Mini till 3rd) Apple produced two-in-one Smart Cases. Pity they are not available (at least yet) for current iPad Mini 4.

No! Unfortunately the cover of iPad Mini 3 does NOT fit the iPad Mini 4. I have tried it when I upgraded from iPad 3 to 4, and it doesn't work. iPad 4 is slightly longer than the iPad 3, and for some reason it seems that the old cover of the iPad 3 doesn't hold well along the magnetic side of iPad 4.

So I wasted $$ on getting that "older" cover for model 3 a few month ago,

I would guess (but that has to be tried) that the new cover for model 4 wouldn't fit well model (3) either.

Most defiantly. It's connected to the left side of the iPad through magnets. They securely hold the Smart Cover in place on the side of your iPad. And it doesn't come off when in typing mode and tapping the screen hard.